Milder phenotypes can be seen if the mutation renders the enzyme partially active.

The mechanism leading to the manifestations seen appears to be the result of a deficiency in cholesterol.

Cholesterol is a critical component of myelin and other CNS proteins

Cholesterol is also a precursor to the sex steroids, estrogen and testosterone, thus hypocholesterolemia results in a deficiency of these hormones.

The derangement of sterol composition in the cell membranes may result in abnormal cell-to-cell interactions in the developing embryo. (? Explains polydactyly and cleft palate and growth deficiency seen in some??)

Diagnosis is usually made based on the recognition of a constellation of characteristic clinical features, with diagnostic confirmation based on measurement of elevated 7DHC in plasma or other tissues.

The cardinal features include:

Prenatal growth deficiency with subsequent failure to thrive

Developmental delay/ mental retardation (moderate to severe)

Poor muscle tone (hypotonia)

Characteristic facies (change with age and are difficult to recognize in adulthood)